We propose to further delineate mechanisms controlling the Ca oscillations in heart muscle cells which are responsible for contractile activity. Ca influx and efflux rates will be measured in isolated heart muscle cells which have leaky sarcolemmae, so that flux reflects function of subcellular organelles. The frequency of beating will be altered by varying temperature of incubation. The correlation of influx or efflux (or both) with beating frequency will indicate the source of contraction-activating Ca as either internal or external. These studies will be repeated with cells preloaded with various amounts of Ca and suspended in media with various (Ca) to determine if (Ca)i/Ca)o influences the source of contraction-activating Ca.